Spectroscopic and Theoretical Studies of the Zn(II) Chelation with Hydroxyflavones

The Zn(II) complexation of three naturally occurring organic compounds (3-hydroxyflavone, 5-hydroxyflavone, and 3',4'-dihydroxyflavone) has been investigated by electronic spectroscopy combined with quantum chemical calculations. These three ligands, which differ in the nature of their chelating site, lead to the formation of a complex of 1:1 stoichiometry. The experimental results show that it is possible to class the three studied sites, according to their chelating power toward Zn(II), in the following way: alpha-hydroxy-carbonyl > beta-hydroxy-carbonyl > catechol. Time-dependent density functional theory (TD-DFT) calculations were performed to obtain the excitation energies and oscillator strengths of the different complexes. Several effective core potentials (Los Alamos and Stuttgart/Dresden) were used for the description of the Zn ion. Calculations were also performed without any pseudopotential, and they give very satisfying results in the simulation of UV-vis spectra of the three complexes. Only the MWB28 ECP leads globally to a good quality description of the spectral features, roughly comparable to that obtained when the 6-31G(d,p) basis set is used to describe the Zn(II) orbitals. The analysis of the results shows that the nature of electronic transitions involved in the UV-vis spectra greatly differs according to the substitution pattern of the flavonoid.